A laser produces light with a wavelength that is determined primarily by the characteristics of the laser's gain medium and the discrete set of resonant modes supported by the laser's optical cavity. A single-mode laser is designed to support just one of the resonant modes of the optical cavity, producing light with a very narrow and well-defined wavelength. Tunable lasers allow the wavelength of the generated light to be controllably tuned over a range of wavelengths. When a single-mode laser is tuned, the wavelength of the laser can discontinuously jump from one resonant mode to the next-an undesired effect called mode hopping. Continuously tunable, single-mode lasers have great value in many current and anticipated technological applications including, for example, wavelength division multiplexed (WDM) fiber optic communication. Creating such lasers, however, has not been without its challenges.
According to a traditional method for tuning a laser, a diffraction grating is used as one of the cavity mirrors of an external cavity laser (ECL). The grating functions as a narrow-bandwidth reflector that tunes the laser output wavelength by controlling the precise position and orientation of the grating. Because mechanical movement of the entire grating is required, the laser cannot tune quickly. It is also sensitive to optical misalignments. Another approach uses a fixed grating to disperse light wavelengths across the surface of a micromirror array. Micromirrors positioned where the desired wavelength illuminates the array are controlled to reflect light back to the grating, while all other mirrors are controlled to deflect light away. Although this approach does not require large mechanical movements for wavelength tuning, it is also sensitive to misalignments. Moreover, it does not tune continuously.
An alternative tunable laser technology is the tunable vertical cavity surface emitting laser (VCSEL). The fabrication technology of the VCSEL, however, limits the power and quality of the output beam. There are various other methods of tuning lasers, including distributed feedback, sampled grating, and distributed Bragg reflector, each one with its own advantages and disadvantages, as illustrated in Table 1.
TABLE 1Advantages and disadvantages of various tunable laser designsTechnologyAdvantagesDisadvantagesClassicalwide tuning range, andthermal and mech. instability,tunablehigh powerslow tuning, andECLlarge sizeTunablesmall size,low power,VCSELsimple packagingoptical pumping,(inexpensive), wide tuningpoor quality optics at 1.5 μm,range, and fast tuningand poor output qualityOtherssimple/reliable structure,complex/unreliable structure,and/or wide tuning rangelow tuning range, and/ormode hopping
There is thus a need for a laser that is compact and mechanically robust, outputs high power, provides single-mode operation, and is quickly tunable over a wide tuning range without mode hopping.